The present invention relates to measuring a transfer function of a device, and more particularly to measuring the transfer function of a frequency analyzer independently from a transfer function for a test signal source.
A network analyzer is used to measure frequency transfer functions of various circuits, such as filters, amplifiers, etc., by using two factors—amplitude and phase. However, it is very difficult for the network analyzer to measure the frequency transfer function of a spectrum analyzer since the spectrum analyzer has down converters that make the measurement complicated.
Previously for measuring complicated transfer functions a first method provides an apparatus under test with a continuous wave (CW) signal as an input having known characteristics (amplitude and phase). A second method measures input and output signals of the apparatus under test to determine the characteristics, the input signal being a known continuous wave (CW) signal that changes in frequency. The second method omits the measurement of phase because the phase change is negligible when the bandwidth is narrow enough.
If a wide bandwidth signal is used for measuring transfer functions, a known modulation signal in place of the CW signal is input to the apparatus under test, and then the input and output signals are measured to determine the transfer function by supposing the known modulation signal is ideal, as shown in U.S. Pat. No. 6,526,365.
As indicated above, the network analyzer is not suitable for measuring the transfer function of a complex apparatus, such as a spectrum analyzer having down converters. The measurement method using a CW signal requires special measurement instruments and cannot measure a phase characteristic. The measurement method using a known modulation signal is not realistic because the method requires an ideal modulation signal source. Also the output signal of the apparatus under test includes a combination of the transfer function G(w) of the signal source and the transfer function F(w) of the apparatus under test. Therefore it is necessary to evaluate the transfer functions G(w) and F(w) independently to determine the transfer function F(w). Even if the test signal is measured by the spectrum analyzer, the result includes the combination of the transfer functions of the spectrum analyzer and the signal source so that this method requires the transfer function of the spectrum analyzer to be accurately measured in advance.
What is desired is a method of measuring the transfer function of a complex apparatus, such as a spectrum analyzer, independently of the transfer function of a test signal source.